Ambipolar organic transistors are technologically interesting because oftheir potential applications in light-emitting field-effect transistors [1]and complementary-metal-oxide-semiconductor (CMOS) devices by providing easeof design, low cost of fabrication, and flexibility [2]. Although commonorganic semiconductors show either n- or p-type charge transportcharacteristic, organic transistors with ambipolar characteristics have beenreported recently. In this work, we show that ambipolar transport can beachieved within a single transistor channel using LiF gate dielectric in thetransistors with pentacene active layer. This ambipolar behavior can becontrolled by the applied source-drain and gate biases. It was found that atlow source-drain biases multistep hopping is the dominant conductionmechanism, while in high voltage regimes I-V data fits in Fowler-Nordheim(F-N) tunneling model. From the slope of the F-N plots, the dependencybetween field enhancement factor and the transition point in conductionmechanism upon gate bias has been extracted. The transition points show moredependency on gate voltage for negative biases compared to the positivebiases. While sweeping negative gate voltages from -5 to -20 V, thesource-drain voltages change from about 27 to 17 V. On the other hand, forpositive gate voltages from 5 to 20 V, the value of the transition pointstays at approximately 36 V. In order to further understand the transportmechanisms, new structures with an interface layer between dielectric andactive layer have been fabricated and characterized. As expected, asignificant decrease in the amount of the source-drain current has beenobserved after introducing the interface layer.